The present invention relates to a safety valve for use in a sealed lead-acid secondary cell, and more particularly, to a safety valve with a prolonged life and improved explosion protection.
Two types of sealed lead-acid secondary cells are known; one uses a catalyst plug, and the other uses the negative electrode to absorb oxygen that has been evolved in the charging cycle. The present invention concerns the second type of sealed lead-acid cell.
The second type of sealed lead-acid secondary cells have many advantages: no water need be added during the life of the cell, no electrolyte will leak, the cell can be used in any position (independent of attitude), and there is no need for occasional equalizing discharge. Because of this maintenance-free feature, sealed lead-acid secondary cells of the type that absorb oxygen by the negative plate are making inroads into various fields such as portable VTRs, portable TV sets, emergency power sources and portable power sources.
The lead-acid secondary cell of the type contemplated by the present invention ensures electrolyte sealing by using the negative electrode to absorb oxygen that has evolved during the charging cycle. Therefore, the pressure in the cell is lower than atmospheric pressure by the difference corresponding to the partial oxygen pressure. This type of cell is equipped with a safety valve that prevents atmospheric oxygen from entering the cell and relieves an abnormal increase in the cell pressure. The safety valve used with this type of cell is typically made of rubber. The proper operation of the cell requires that the safety valve made of rubber (hereunder referred to as a rubber valve) opens and closes at predetermined pressures. In other words, a deteriorated rubber valve will cause serious damage to the cell performance. For example, if the rubber valve sticks to its valve seat, the valve opening pressure increases to cause bulging of the cell. If the valve closing pressure drops, atmospheric oxygen will enter the cell to decrease its capacity.
Therefore, one object of the present invention is to provide a sealed lead-acid secondary cell equipped with a safety rubber valve whose opening and closing pressures are maintained constant throughout the cell service.
This object can be achieved by maintaining a supply of an oil to a rubber valve from a porous member that is impregnated with the oil and which is disposed around or below the valve. Alternatively, the rubber valve may be submerged in a liquid substance so as to prevent oxidative deterioration of the rubber. In a third embodiment, two or more rubber valves may be connected in series in such a manner that the valve positioned closer to the cell element opens at a higher pressure whereas the valve positioned farther away from the element opens at a lower pressure. The rubber valve farther away from the element has a relatively small tensile stress and undergoes only a small amount of plastic deformation. Furthermore, the life of that rubber valve is extended since it has a reduced chance of direct contact with sulfuric acid mist.
These are current trends for sealed lead-acid secondary cells to be incorporated in sophisticated electronic equipment and for larger-size cells to be employed. This requires utmost care in protecting the cells from explosion, but none of the commercially available sealed lead-acid cells feature complete protection against explosion. If the constant-voltage charging of the cell is normal, the predominant gas evolving in the cell is hydrogen and the presence of oxygen that might cause an explosion is negligible. However, if the cell is overcharged at a rate exceeding the ability of the negative electrode to absorb evolving oxygen, or if the battery charger fails and the cell is kept charged with excessively large currents, a high volume of oxygen is evolved to cause a pressure buildup in the cell. The safety valve opens and the volumetric ratio of hydrogen to oxygen in the cell becomes 2:1. If there occurs a spark or other sources of ignition in the neighborhood of the cell, it blows.
It is therefore very important to provide an explosion protection around the safety valve from which oxygen escapes. However, rubber of which the safety valve is made unavoidably deteriorates with time. Another cause of rubber deterioration is an electrolyte splash. If the rubber becomes deteriorated, the safety valve does not close sufficiently to prevent the formation of lead sulfate at the negative electrode. If this occurs, the cell discharges by itself and its capacity drops. The life of the cell using a deteriorated rubber valve can be extended by replacing it with a new valve.
Therefore, another object of the present invention is to provide a sealed lead-acid secondary cell which is explosion-proof and has a prolonged life. This object can be achieved by equipping the gas vent with a detachable explosion-preventing filter which covers the rubber valve. The filter renders the cell explosion-proof and extends its life by permitting easy valve replacement.